Undiscovered Oil in Five Rift Basins of Mongolia and China: A USGS Assessment

U.S. Geological Survey Fact Sheet 2018–3041, National and Global Petroleum Assessment. By the Mongolia and China Basins Assessment Team: Christopher J. Schenk, Tracey J. Mercier, Janet K. Pitman, Thomas M. Finn, Cheryl A. Woodall, Kristen R. Marra, Phuong A. Le and Heidi M. Leathers-Miller.

Using its geology-based method, the USGS estimates mean undiscovered, technically recoverable conventional resources of 187 million barrels of oil and 85 billion cubic feet of gas in the East Gobi, Nyalga, Tamtsag-Hailar, Erlian and Yingen basins of Mongolia and China.

The five assessment units. Base map from the National Park Service. USGS.

The basins

The five basins — each really a group of subbasins — share a similar history:

• Rifting. Stretching in the Middle Jurassic and again in the Late Jurassic–Early Cretaceous opened a regional system of basins, filled with river, delta and lake-edge sandstones and with shallow- to deep-water lake shales. The deep-water lake shales are the source rocks.
• Sag. After rifting stopped, the basins kept slowly sinking, burying the source rocks deeper so that locally they matured and generated oil that migrated.
• Inversion. In the Late Cretaceous, regional compression, uplift and erosion inverted many of the rift structures, largely halted maturation, and may have moved oil out of traps or let it escape.

What was assessed

One Mesozoic Composite Total Petroleum System, with an assessment unit in each basin covering all potential undiscovered conventional oil fields charged by the lake source rocks — Lower Cretaceous rift shales and shales such as the Middle Jurassic Eedemt Formation.

• Source rocks vary from basin to basin, but contain Type I kerogen (some Type III), are up to 700 metres thick, hold up to 10 weight percent organic carbon, and have hydrogen index values up to 600 milligrams of hydrocarbon per gram of organic carbon.
• Model. Oil formed where the lake shales reached the oil window, migrated into rift reservoirs and was trapped — perhaps only to move again, or be lost, during the Late Cretaceous inversion.
• Uncertainty lies in how widespread the source rocks are within the grabens, how mature they are, and how much oil the traps kept through compression and inversion.

Results

By basin (mean; oil in million barrels, gas in billion cubic feet):

In total, the mean is 187 million barrels of oil (F95–F5 range 0 to 453) and 85 billion cubic feet of associated gas (0 to 212). About 86 percent of the oil — 162 million barrels — is estimated to lie in the Tamtsag-Hailar and Erlian basins.

Sources

• Schenk, C.J., and others, 2018, Assessment of undiscovered conventional oil resources of the East Gobi, Nyalga, Tamtsag-Hailar, Erlian, and Yingen basins of Mongolia and China, 2018: U.S. Geological Survey Fact Sheet 2018–3041. https://doi.org/10.3133/fs20183041 · https://pubs.usgs.gov/publication/fs20183041
• The map, the results by basin and the assessment team come from the fact sheet's PDF.
• Rewritten in hubnx's own words.